Hydroprocessing can include processes which convert hydrocarbons in the presence of hydroprocessing catalyst and hydrogen to more valuable products.
Hydrocracking is a hydroprocessing process in which hydrocarbons crack in the presence of hydrogen and hydrocracking catalyst to lower molecular weight hydrocarbons. Depending on the desired output, a hydrocracking unit may contain one or more fixed beds of the same or different catalyst. Slurry hydrocracking is a slurried catalytic process used to crack residue feeds to gas oils and fuels.
Due to environmental concerns and newly enacted rules and regulations, saleable fuels must meet lower and lower limits on contaminates, such as sulfur and nitrogen. New regulations require essentially complete removal of sulfur from diesel. For example, the ultra low sulfur diesel (ULSD) requirement is typically less than about 10 wppm sulfur.
Hydrotreating is a hydroprocessing process used to remove heteroatoms such as sulfur and nitrogen from hydrocarbon streams to meet fuel specifications and to saturate olefinic compounds. Hydrotreating can be performed at high or low pressures, but is typically operated at lower pressure than hydrocracking.
Hydroprocessing recovery units typically include a stripping column for stripping hydroprocessed effluent with a stripping medium such as steam to remove unwanted hydrogen sulfide. The stripped effluent then is heated in a fired heater to fractionation temperature before entering a product fractionation column to recover products such as naphtha, kerosene and diesel.
Hydroprocessing and particularly hydrocracking is very energy-intensive due to the severe process conditions such as the high temperature and pressure used. Over time, although much effort has been spent on improving energy performance for hydrocracking, the focus has been on reducing reactor section heater duty through efficient heat exchange network design. However, a large heater duty is required to heat stripped effluent before entering the product fractionation column to separate diesel from unconverted oil.
Newly enacted rules and regulations impose boiling point ranges on commercial diesel. Euro IV and V diesel specifications require diesel product to have a T95 at 360° C. meaning that 95 vol % of the diesel stream must boil off when it is heated to 360° C. To meet this specification, a conventional product fractionator requires a large heat input, a large number of trays and more operating expense to effect the separation.
There is a continuing need, therefore, for improved methods of recovering fuel products from hydroprocessed effluents. Such methods must be more energy efficient and less capital intensive to meet the increasing needs of refiners.